Existing digital data transmission systems such as DAB (Digital Audio Broadcast) or DVB (Digital Video Broadcast) are based on OFDM modulation as described in “Das Obertragungsverfahren des zukünftigen digitalen Hörrundfunks” [Transmission Method of the Future Digital Audio Broadcast] by Wächer, Der Fernmeldeingenieur 11 and 12/92, pp. 1-43; and “OFDM Obertragungsverfahren fur den digitalen Fernsehrundfunk” [OFDM Transmission Method for Digital TV broadcast] by Engels, Rohling, and Breide at pp. 260-270. To make, Rundfunktechnische Mitteilungen 1993, pp. 260-270). To make detection of transmission frames possible, a zero symbol is used in these systems, i.e., no power or very little power is transmitted. In such pure distributed services, the zero symbol only occurs once and therefore its detection in the receiver is simple and unambiguous.
PCT Patent Application No. 97/29568 describes frequency and time synchronization, wherein a combination of a wattless zero symbol, two symbols S1 and S2 that have signals at different frequencies for determining a frequency offset and a time offset, and two symbols S3 and S4 for symbol synchronization, is used. The frequency offset and the time offset are determined using a maximum likelihood estimation, which includes complex digital signal processing, in particular multipliers.
“A Frequency and Timing Period Acquisition Technique for OFDM Systems,” presented by H. Nogami et al. at IEICE Transactions on Communications Japan on Aug. 1, 1996, describes the combination of a zero symbol and a pilot for frequency and time synchronization at pp. 1135-1146. The zero symbol is used for the first rough time synchronization. The pilot has a sequence of frequencies in which signal power is transmitted. This sequence is a pseudo-random sequence.
In European Patent No. 0 722 235, time synchronization is achieved by applying a correlation to a signal S0, which has signal power for a pseudo-random sequence of frequencies. For this purpose, reference timing generator 62 is used (Col. 18, lines 18-24). Signal S0 is furthermore used for frequency synchronization. Signal S0 has a symmetrical power distribution as a function of the frequency. A detected asymmetry indicates erroneous synchronization regarding frequency.
“Low-Overhead, Low-Complexity Burst Synchronization for OFDM,” presented by T. M. Schmidl et al. at the 1996 IEEE International Conference on Communications (ICC) entitled Converging Technologies for Tomorrow's Applications June 23-27, describes a time synchronization in which a pilot is split in time into two identical halves at pp. 1301-1306. The signal power is distributed to the straight frequencies using a pseudo-random sequence, while the non-straight frequencies remain wattless. According to this paper, an intermediary result is obtained by multiplying a signal value from the first half by the conjugated complex of the corresponding signal value in the second half, then adding these products and dividing the result by the signal power of the second half. Time synchronization is achieved by searching for a maximum.
German Patent No. 4319216 describes that symbols can be transmitted for synchronization purposes; such symbols can be detected in the receiver on the basis of their correlation properties, in particular their self-correlation properties using correlation methods. The correlation method has the disadvantage that it is relatively complicated to implement in the receiver due to the high number of complex multiplications, resulting in high power consumption.
In a communication system in which the transmission channel can be used alternatingly by a base station and by mobile users, as is the case with mobile wireless systems, the zero symbol, which is suitable for pure distributed services for frame synchronization, is ambiguous because prior to the beginning of the transmission from the base station into the transmission channel (downlink) and the beginning of the transmission from a mobile subscriber to the transmission channel (uplink), a Transceiver Turnaround Interval without transmitting power arises and, in addition, the time slots for uplink signaling are not necessarily occupied, since some subscribers may access the transmission channel in random access.
Since the detection of the beginning of a frame, in particular in a wireless TDMA (time division multiple access)-TDD (time division duplex) multicarrier transmission system, is particularly important because the time between two downlink phases (frame time) can vary due to a flexible organization of the multiple access by the subscribers via the downlink, the disadvantages of the correlation method have to be taken into account.